I. Field Of The Invention
The present invention generally relates to the field of electronic circuit testing and, more particularly, to the field of in-circuit and functional testing of digital devices on electronic circuit boards.
II. Related Art
As the use and complexity of digital circuits has increased over the last decade, so also has the need to test such circuits increased in order to ensure proper operation. Basically, two types of digital circuit test techniques have been developed, namely, so-called "in-circuit" test techniques and so-called "functional" test techniques.
In functional test techniques a known digital pattern is applied to the circuit inputs, and a comparison is made of the circuit output with an expected output. The differences between the actual and expected outputs provides an indication of circuit operation. Unfortunately, this technique is only useful when it is desirable to know the overall operation of a circuit, for example, a circuit which has been assembled on a printed circuit board. Very often, it is necessary to test individual circuit elements or groups of elements which have been assembled on a printed circuit board apart from the overall circuit operation.
In the in-circuit testing techniques, testing is performed on a circuit element or elements isolated from the remainder of the circuit. In-circuit testing techniques generally involve the application of a preselected digital pattern to the input of an individual circuit element, a so-called device under test or "DUT", and a comparison of the DUT response to an expected response. Since the circuit element(s) under test typically are connected to other circuit elements, it is necessary to overdrive any digital pattern or signal which is being applied by an "upstream" circuit element or logic device. Upstream logic devices are those devices whose outputs normally drive the inputs of the DUT. An overdrive signal is a signal which is superimposed at a selected location in a circuit.
With regard to in-circuit testing, several devices have been described. U.S. Pat. No. 3,781,689 to Marshall et al., discusses a pulse generator for an in-circuit tester. The generator can produce three output states for use in an in-circuit probe. The three output states are a logic high ("1"), a logic low ("0") and a high resistance state, the so-called "three-state", or "off state". FIG. 1 is a diagrammatic representation of the three states, which can be applied to the DUT by the in-circuit probe.
U.S. Pat. No. 3,965,468 to Bronson, describes a probe for use in an in-circuit testing system where various logic patterns or signals are generated, and a flashing light indicates the particular pattern or signal being applied. While hand-held probe-type devices, or systems incorporating hand-held probes, are convenient in-circuit testing tools, they are too time consuming if many tests are required. For example, if in-circuit testing was performed on a complex printed circuit board for quality control purposes, a hand held probe approach to in-circuit testing would be too time consuming.
To perform multiple simultaneous in-circuit tests on several individual circuit elements mounted on a single printed circuit board, test devices were developed such as that disclosed in U.S. Pat. No. 4,588,945 (hereafter, "the '945 patent") to Groves et al. The test device taught by the '945 patent was especially effective in finding faults which occur most commonly during circuit board assembly, i.e. solder shorts, wrongly inserted components, damaged or marginal components, and missing components.
In such test devices a printed circuit board having circuit elements mounted thereon is in turn mounted or affixed to a so-called "bed of nails." Each nail acts as an individual probe either providing a preselected test signal to or receiving an output signal from a DUT. As described in the '945 patent, a controller module applies multiple pre-generated signal patterns to multiple DUTs through a driver module. The DUT responses are received through a sensor module and compared to expected responses. The driver module is made up of a multiplicity of identical driver circuits which generate the actual voltage signals provided to selected probes, or nails. Each circuit can provide logic high, logic low, and three-state (off).
In U.S. Pat. No. 4,998,026 (hereafter, "the '026 patent") by King (the inventor herein), a driver circuit operated with any order of three state logic. The driver circuit taught by the '026 patent was unique because the driver circuit could operate at high speed and at high/low currents with unusually accurate results when driving a DUT. However, while exhibiting excellent functionality, the driver circuit taught by the '026 patent employs complex circuitry with many parts, resulting in high costs and large space requirements.